1. Field of the Invention
The invention is concerned with manufacture of high grade viscous oil products from crude petroleum fractions. It is particularly directed to the manufacture of high quality lube base stock oils from crude stocks of high wax content, commonly classified as "wax base" as compared with the "naphthenic base" crudes. The latter crudes are relatively lean in straight chain paraffins and yield viscous fractions which inherently possess low pour points.
2. Description of the Prior Art
High quality lube base stock oils are conventionally prepared by refining distillate fractions or the residuum prepared by vacuum distilling a suitable crude oil from which the lighter portion has been removed by distillation in an atmospheric tower. Thus, the charge to the vacuum tower is commonly referred to as a "long residuum", and the residuum from the vacuum tower is distinguished from the starting material by referring to it as the "short residuum".
The vacuum distillate fractions are upgraded by a sequence of unit operations, the first of which is solvent extraction with a solvent selective for aromatic hydrocarbons. This step serves to remove aromatic hydrocarbons of low viscosity index and provides a raffinate of improved viscosity index and quality. Various processes have been used in this extraction stage, and these employ solvents such as furfural, phenol, sulfur dioxide, and others. The short residuum, because it contains most of the asphaltenes of the crude oil, is conventionally treated to remove these asphalt-like constituents prior to solvent extraction to increase the viscosity index.
The raffinate from the solvent extraction step contains paraffins which adversely affect the pour point. Thus, the waxy raffinate, regardless of whether prepared from a distillate fraction or from the short residuum, must be dewaxed. Various dewaxing procedures have been used, and the art has gone in the direction of treatment with a solvent such as MEK/toluene mixtures to remove the wax and prepare a dewaxed raffinate. The dewaxed raffinate may then be finished by any of a number of sorption or catalytic processes to improve color and oxidation stability.
The quality of the lube base stock oil prepared by the sequence of operations outlined above depends on the particular crude chosen as well as the severity of treatment for each of the treatment steps. Additionally, the yield of high quality lube base stock oil also depends on these factors and, as a rule, the higher the quality sought, the less the yield. In general, naphthenic crudes are favored because less loss is encountered, particularly in the dewaxing step. In many cases, however, waxy crudes are more readily available, and it would be desirable to provide a process for preparing high quality lube base stock oils in good yields from such waxy crude oils.
In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113.
In U.S. Pat. Re. No. 28,398 is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938.
In copending patent application Ser. No. 817,309 filed July 20, 1977 (now U.S. Pat. No. 4,137,148) is described a process for preparing specialty oils of very low pour point and excellent stability from a waxy crude oil distillate fraction by solvent refining, catalytic dewaxing over a zeolite catalyst such as ZSM-5, and hydrotreating, under specified conditions. The entire contents of that patent application are incorporated herein by reference.
It is an object of this invention to provide a process for preparing a high quality lube base stock oil having a pour point not greater than +30.degree. F. from a waxy crude oil. It is a further object of this invention to provide a process for preparing a high quality lube base stock oil having a pour point of about -25.degree. F. to +30.degree. F. from a waxy crude oil in high yield and with recovery of valuable paraffin wax. Other objects will be evident to those skilled in the art upon reading the entire contents of this specification including the claims thereof.